The present invention relates generally to a method of forming a plurality of objects from a plurality of superimposed sheets. More particularly, it pertains to the method of producing multiple articles from a plurality of sheets of material, such as foamed polystyrene, which generally fuse when heated and pressed in a forming mold.
The preferred embodiment of the present invention is described particularly with relation to thermoplastic materials such as foam polymers (polystyrene, polyethylene, polypropylene) and solid polymers (polystyrene, polyolefins, polyester), but it is to be understood that it is equally applicable to other formable materials which generally fuse contiguous faces thereof under the contact times, temperatures and pressures involved in the forming process. A typical material used in the instant method is foamed polystyrene sheet, which may have a laminated skin, an extrusion coated skin, or a skin formed on its surface by a process, described in detail in the U.S. Pat. No. 3,311,681 issued to Cherney, et.al.
Further, the term "sheet" as used herein is meant to designate an article having two surfaces larger in area than any other single surface of the article. This definition is meant to include thin, flexible sheets of foam plastic, either flat (i.e., the planes in which each large surface lies are parallel to one another) or in some other shape.
Flexible foamed polystyrene sheet is typically produced by heating a mixture of polystyrene and a foam cell size controlling additive (e.g., talc or a mixture of citric acid and sodium bicarbonate) to a fluid, plastic state in an extruding machine. A liquid which will vaporize to form a gas at extrusion die temperatures and atmospheric pressure is added to the mixture. This expansible fluid mass is then extruded through an orifice or die opening where foaming of the plastic takes place to form a foamed polystyrene sheet.
In many applications, a skin is provided on the surface of the foam to give the foam a superior outer appearance, provide resistance to surface abrasion or add structural strength. Numerous techniques have been described in the prior art for producing skins on the foam, such as combining dual melt streams, one of which contains a foaming agent (developed by National Rubber Machine a Corporation of Tallmadge, Ohio); see U.S. Pat. No. 3,972,664 issued to Fillmann. Another approach includes surface chilling of the foam during the extrusion process. In the Cherney method, an air ring is provided proximate the extrusion orifice to chill one surface of the foam sheet as it is extruded to create a skin thereon. (U.S. Pat. No. 3,311,681 issued to Cherney et.al.) Most recently, in applicant's co-pending application, Ser. No. 264,574, filed May 18, 1981, means are disclosed for producing a skinned foam wherein a nucleating agent is omitted from one stream of a multiple polymer stream, and temperature control of said stream is provided to prevent homogeneous nucleation.
In the industry, these thermoformable thermoplastic materials are generally extruded and the resulting sheets wound into feeder rolls. Typically this rolled sheet will be mounted on an unwind stand and arranged to feed a thermoformer such as the "THERMOPHASER", manufactured by Irwin Research and Development, Inc. From the unwind stand it is fed to a heat tunnel having spaced heating elements positioned therein. Once heated it is selectively and incrementally advanced to a press mold where the deformable sheet is press formed into the desired object. In the final sequence, generally referred to as the trimming operation, the sheet is selectively moved to a trimming apparatus where dies cut the finished product from the stock. Other thermoforming systems available for similar use are the CMS Thermoformer from Gloucester Engineering Company, Inc., and the Continuous Thermoforming Machine from Brown Machinery, a Leesona Co.
It is a characteristic of the thermoplastic polymers that the application of heat softens the material. In this softened state, these materials will commonly fuse with other contiguous surfaces. For a particular material pair, a time, temperature and pressure relationship determines the fusing tendency. This property is used to advantage in many commercial applications. For example, sheets of paper, metal foil, or polystyrene can be fused to polystyrene foam sheet by applying heat and pressure for sufficient time. Twin sheet forming of hollow products has been accomplished in other industries such as in the formation of gas tanks where two sheets of material are heated separately and formed in a hollow mold. A gas is used to form the two sheets against the opposite mold surfaces, while mold pressure outside the cavity area fuses the sheets where they contact each other.
Materials that will not fuse together have been formed in multiple sheets; for example, paper plates are molded and trimmed from multiple sheets. However, since multiple layers of foam are generally fused together when an elevated temperature is combined with applied pressure under usual pressure molding conditions, it has been a practice in the prior art to feed these plastic materials through the thermoformer machines one sheet at a time. Multiple sheet forming of fusible materials has generally not been attempted due to the presumed risk of fusing the sheets during the hot forming process even though multiple sheet forming clearly presents an economical and efficient method for increasing the capacity of the machinery employed.
In the prior art, the multiplicity of parts per stroke required was commonly achieved with a multiplicity of die cavities. When fed with a single sheet, multiple parts per stroke were achieved but only one part per cavity. With the present invention a multiplicity of die cavities may still be used, resulting in multiple parts per stroke as well as multiple parts per cavity.